The Deuteron

The deuteron, composed of a proton and a neutron, is a stable particle. As an atom, it is called deuterium and as an isotope of hydrogen it has an abundance of 1.5 x 10-4 compared to 0.99985 for ordinary hydrogen. It's stability is remarkable since the free neutron is unstable, undergoing beta decay with a halflife of 10.3 minutes. The measured binding energy of the deuteron is 2.2 MeV.

If the neutron in the deuteron were to decay to form a proton, electron and antineutrino, the combined mass energies of these particles would be

2(938.27 MeV) + 0.511 MeV = 1877.05 MeV

But the mass of the deuteron is 1875.6 MeV, implying that, upon energy grounds, it is stable agains such a decay. The free neutron yields an energy of 0.78 MeV in beta decay, but the 2.2 MeV binding energy of the deuteron prevents its decay.

The stability of the deuteron is an important part of the story of the universe. In the Big Bang model it is presumed that in early stages there were equal numbers of neutrons and protons since the available energies were much higher than the 0.78 MeV required to convert a proton and electron to a neutron. When the temperature dropped to the point where neutrons could no longer be produced from protons, the decay of free neutrons began to diminish their population. Those which combined with protons to form deuterons were protected from further decay. This is fortunate for us because if all the neutrons had decayed, there would be no universe as we know it, and we wouldn't be here!